TUESDAY April 29, 2014, 2014 -- Major lung cancer surgery in the United States has become safer in recent decades, especially at hospitals that perform high numbers of these operations, a new study says.

Researchers analyzed data from more than 121,000 patients who had major lung cancer surgery between 2007 and 2011. Death rates were 2.8 percent at 30 days after surgery and 5.4 percent at 90 days after surgery.

The higher death rate after 90 days was a surprise, according to the study, scheduled for presentation Tuesday at the annual meeting of the American Association for Thoracic Surgery in Toronto.

"This increase in the number of deaths between 30 days and 90 days after surgery was not expected and has not been extensively reported in the past, as mortality rates after surgery are traditionally examined at 30 days," lead author Dr. Christopher Pezzi, a surgeon at Abington Health in Pennsylvania, said in a thoracic society news release.

"The reasons for this ongoing mortality beyond 30 days are not yet clear, but deserve further investigation," Pezzi added. Research is typically viewed as preliminary until published in a peer-reviewed journal.

The operations were performed at 1,200 U.S. hospitals. The researchers found that the overall death risk was lower in hospitals that did more than 90 major lung cancer surgeries a year than at those that did fewer than 10 such surgeries a year, 1.7 percent vs. 3.7 percent.

More than 10,000 of the surgeries included in the study were performed at lower-volume hospitals.

Other factors associated with increased risk of death included: older age, being male, more advanced tumors, a higher number of serious medical conditions before surgery, and being poor.

Having private health insurance was associated with a low (1.8 percent) death rate 30 days after surgery.